1. Field of the Invention
The present invention relates to a light emitting device using an organic compound, and more particularly to a novel compound having a specific molecular structure and an organic electroluminescence (EL) device using the same.
2. Related Background Art
In an old example of an organic light emitting device, a voltage is applied to an anthracene evaporated film to emit light (Thin Solid Films, 94 (1982), 171). However, in recent years, applied research including material development has been vigorously made for making a light emitting device which can easily have an increased area as compared to an inorganic light emitting device, which can provide desired color development owing to development of various new materials, which has, for example, an advantage in that it can be driven at a low voltage, and which has high speed responsiveness and high efficiency.
For example, as detailed in Macromol. Symp. 125, 1 to 48 (1997), an organic EL device is generally structured to have two (upper and lower) electrodes formed on a transparent substrate and an organic substance layer including a light emission layer formed between the electrodes.
In addition, investigation has been recently made into a device using not only light emission utilizing fluorescence upon transition from a singlet exciton to a ground state but also phosphorescence emission via a triplet exciton typified by Improved energy transfer in electrophosphorescent device (D. F. O'Brien et al, Applied Physics Letters Vol 74, No 3, p 442 (1999)) and Very high-efficiency green organic light emitting devices based on electrophosphorescence (M. A. Baldo et al, Applied Physics Letters Vol 75, No 1, p 4 (1999)). In each of those documents, an organic layer having a four-layer structure is mainly used. The structure is composed of a hole transport layer, a light emission layer, an exciton diffusion prevention layer, and an electron transport layer from an anode side. The materials used are carrier transport materials and a phosphorescence emission compound Ir(ppy)3 shown below.

In addition, light emission from ultraviolet to infrared can be performed by changing the kind of a fluorescent organic compound. In these days, research has been actively made on various compounds.
In addition to organic light emitting devices using such low-molecular materials as those described above, a group of the University of Cambridge has reported organic light emitting devices using conjugate polymers (Nature, 347, 539 (1990)). This report has confirmed that light emission can be obtained by a single layer by forming polyphenylene vinylene (PPV) into a film according to a coating system.
As described above, recent progress of an organic light emitting device is remarkable, and is characterized in that a high responsive, thin, and lightweight light emitting device that can be driven at a low applied voltage and provides high luminance and a variety of luminous wavelengths can be made. Therefore, the progress proposes the potential of the organic light emitting device to find use in a wide variety of applications.
However, at present, an optical output having higher luminance, or higher conversion efficiency has been required. In addition, there still remain a large number of problems in terms of durability such as a change with time due to long-term use and deterioration due to an atmospheric gas containing oxygen or to moisture. Furthermore, light emission of blue, green, or red having a good color purity is necessary when application to a full-color display or the like is attempted. However, those problems have not been sufficiently solved yet.
In addition, a large number of aromatic compounds and condensed polycyclic aromatic compounds have been studied as fluorescent organic compounds used for an electron transport layer, a light emission layer, and the like. However, it is hard to say that a compound sufficiently satisfying light emission luminance and durability has been already obtained.
Examples of a patent document describing application of a fluorene compound relating to the present invention to an organic EL include Japanese Patent Application Laid-Open No. 2004-43349, International Publication No. WO 99/54385, and Japanese Patent Application Laid-Open No. 2003-229273. However, none of the patent documents discloses an organic compound of the present invention characterized by including a partial structure containing a fluorene ring and a phenylene ring on a straight line in a molecular structural formula. In addition, a fluorene compound has been reported as application to laser coloring matter (Journal of Fluorescence, Vol. 5, No. 3, 295 (1995)).
An organic EL device must have an optical output of high efficiency and high luminance and sufficiently secure high durability when the device is to be applied to a display device such as a display. However, such an optical output and high durability have not been sufficiently achieved.